1. Field of the Invention
This invention relates to gamma-ray imaging systems as used in nuclear medicine. In a primary application the invention relates to three-dimensional tomographic imaging of an administered isotope in the body.
2. Description of Prior Art
Existing nuclear medicine cameras suffer from three primary difficulties; low collection efficiency, poor lateral resolution and a lack of depth delineation. These difficulties stem primarily from the imaging structure. Since these high energy photons cannot be imaged with a lens, pinhole and parallel hole collimators are used with their resultant low collection efficiency. The collection efficiency can be improved somewhat by increasing the size of the pinholes, with a resultant reduction in lateral resolution. In addition, these imaging systems inherently produce a single projection image of the source, with no depth information. The various source planes parallel to the detector are essentially compressed into a single image.
A number of methods have been considered to overcome one or more of these problems. One of these is the use of a multi-aperture plate which provides a coded version of the image. The image is then decoded to provide the desired three-dimensional source distribution. Such a system is described in a paper by H. H. Barrett entitled, "Fresnel Zone Plate Imaging in Nuclear Medicine" in the Journal of Nuclear Medicine, Vol. 13, pages 382-285, 1972. This system, and many similar versions of it, have been found to provide an improved signal-to-noise ratio only for very small sources. For larger sources the performance becomes significantly worse than that of a single pinhole. In addition, the depth delineation is blurred with the information of adjacent planes appearing in the background.
Another system for providing three-dimensional reconstructions was described by T. Budinger and G. T. Gullberg entitled, "Three-Dimensional Reconstruction in Nuclear Medicine Emission Imaging," in the IEEE Transactions on Nuclear Science, NS-21, 1974. This system used a standard nuclear medicine camera to collect an array of projection images from different angles. This projection information is then used to provide a three-dimensional reconstruction using mathematical techniques identical to those used in radiographic computerized axial tomography systems such as those of EMI. Although these systems provide excellent three-dimensional reconstructions they do so at greatly increased dosage to the patient. The efficiency of each projection is unchanged over conventional systems, and the increased number of projections requires increased dosage. In addition, since the projections are taken in sequence, images of moving organs will be smeared because of the long data acquisition time.
A similar system was described by L. T. Chang, B. Mac Donald and V. Perez-Mendez entitled, "Axial Tomography and 3-Dimensional Image Reconstruction," in the IEEE Transactions on Nuclear Science, NS-23, pages 568-572, 1976. Here the sequence of views are taken using a single pinhole in different positions. As before, the collection efficiency is not improved since the various views or projections are taken one at a time. This paper, however, does introduce a Fourier transform reconstruction method for the array of pinhole views.
A system which provides the desired three-dimensional imaging with significantly improved collection efficiency is described in U.S. Pat. No. 3,840,747 issued Oct. 8, 1974 to Albert Macovski. In this patent an array of pinhole apertures are used to provide different views of the source. Rather than opening these in sequence, as in the system described by Chang, the various apertures are modulated, or turned on and off, with shutters. Each aperture is modulated with a different orthogonal function. The superimposed projection images on the detector are recorded as a function of time. These are then decoded, using the same orthogonal functions, to produce a separated array of different views of the source. These are used, as previously described, to reconstruct a three-dimensional image of the source. However, most importantly, since a number of views are collected simultaneously the collection efficiency is significantly improved and the data acquisition time is greatly reduced.
This system of U.S. Pat. No. 3,840,747, however does not indicate a method for choosing the orthogonal modulation functions to the specific requirements of the source being viewed. Thus certain types of sources, such as those having relatively large areas, provide poorer resultant signal-to-noise ratio than systems which acquire the views in sequence. In addition, this patent did not show a system which provided a complete array of views for accurately reconstructing a cross-sectional slice of the source.